Abrasive article and method of manufacturing the same



Patented Aug. 17, 1943 ABRASIVE ARTICLE AND METHOD OF MANUFACTURING THESAME Norman Robie, Niagara Falls, N. Y., assignor to The CarborundumCompany, Niagara Falls, N. Y., a corporation of Delaware N Drawing.Application October 30, 1939. Serial No. 301,980

7 Claims.

This invention relates to abrasive articles including bonded articlessuch as abrasive wheels and stones and coated articles such as abrasivepaper and cloth and flexible abrasive discs. More specifically, theinvention is concerned with such articles where there is employed as a.binder either in whole or in part a resinous polymer including asubstantial proportion of an ester of an acrylic acid.

This application is a continuation-impart of my copending applicationSerial No. 92,937, filed July 27, 1936, and also my copendingapplication Serial No. 100,920, filed September 15, 1936.

An object of the invention is the provision of abrasive articlescomprising improved binders. Another object of the invention is toprovide an abrasive article containing a binder which ineludes as anessential ingredient a resinous polymer including a substantialproportion of an ester of an acrylic acid. Another object of theinvention is the production of abrasive articles in which the bindercomprises a substantial proportion of a. resinous polymer which includesin the polymer an ester of methacrylic acid. Another object or theinvention comprises an improved method for manufacturing abrasivearticles in which the binder includes a resinous polymer. Other objectsof the invention will appear from the description of the invention whichfollows.

Broadly speaking, my improved binders include resins formed bypolymerizing monomeric esters of acrylic acid, either alone orconiointly with other monomeric compounds which include the ethenoidlinkage. Such compounds may be represented by the formula CHz=CRR' and,as an example, suitable binders may be made by polymerizing methylmethacrylate in which compound R is CH3 and R. is COOCHa. Other suitablepolymers of methacrylic acid include the esters of propyl and butylalcohol and particularly the iso and secondary alcohols. For example,the polymer oi the secondary butyl al cohol ester oi methacrylic acidhas properties which make it particularly well adapted as a binder forabrasive products.

As has been stated, my binders may also be prepared by co-polymerizingtwo or more of the acrylic acid esters, or one or more of these esterswith either acrylic acid or with vinyl compounds. A specific compositionwhich I have found to be quite satisfactory for many purposes is theconjoint polymer of the methyl and secondary butyl alcohol esters ofmethacrylic acid. Generally speaking, the esters of methacrylic acid arepreferable to those of acrylic acid CH2=CHCOOH although for somepurposes, where the articles are to be used wet, the lower softeningpoint of esters of acrylic acid are not objectionable and such estersmay be usefully employed. A second example of the conjoint polymerscomprises the co-polymer of methyl methacrylate and vinyl chloride. Bothof these compounds contain the double bond which opens up under suitableconditions to join a plurality of the monomers into a long chain polymerwhich is resinous in character. The monomers themselves are liquidmaterials but when polymerized, form solids having differing properties,depending upon the composition of the monomer or monomers em ployed andthe number of monomers joined into each polymeric molecule.

In making abrasive articles with my improved binders, the polymers maybe formed before admixture with or application of the abrasive grains,or a. part or all of the acrylic compounds may be added in the monomericcondition and polymerized with the abrasive grains in place. Forexample, an abrasive wheel may be prepared by first wetting the abrasivegrains with a monomeric compound or a. mixture of such compounds, apolymer suitably pulverized may then be added to the moistened grains,the article suitably shaped, and then a reaction be brought about topolymerize the liquid monomeric materials to provide a completelysolidified binder.

As an alternative, some of the polymeric materials may be dissolved insuitable monomers and substituted for the monomer in the process justdescribed. By using a solution of polymers in the monomer, a thickerliquid may be obtained which is desirable in some instances andparticularly where the abrasive grains are of coarser grits. Likewise,in the preparation of coated abrasive articles, suitable backings suchas webs of paper of the class commonly used in the production ofsandpaper, may be first coated with a solution of a polymer in amonomer, or a mixture of monomers, abrasive grains distributed upon theliquid coated web in the conventional manner and this abrasive coatedweb may then be suitably treated to solidify the liquid and form theso-called making binder for the abrasive grains. In accordance with theusual practice, it is generally desirable to apply a second coat ofadhesive over the abrasive grains to attach them more firmly to thebacking. The sizing speaking, the esters of acrylic acid are coat may bea liquid similar to that used in the making coat, although it isgenerally preferable to use a somewhat less viscous adhesive for thesizing coat than is employed in the making coat. The making coat of theabrasive grain may be conveniently applied in the usual making machineand the sizing coat may also be applied by the use of rolls or may besprayed on or otherwise applied to the abrasive grains.

Alternatively, in making either the coated or bonded type of article,solutions of polymeric materials in suitable solvents may be used to provide liquids. In another modification, which is of use particularly inthe preparation of bonded articles, the solidified resinous compositionmay be admixed with the abrasive grains and heated to soften itsufficiently to cause adhesion to the grains and thus provide a mixtureof the grains and the binder which can be conveniently molded. Anabrasive article may be then formed by placing such mix in a mold andapplying suitable pressure, usually accompanied by heat to cause afiuxing of the resin according to the usual hot molding methods whichcauses the adhesion of the bond to the grains and the unification of thegrains into a finished article which can be removed from the mold uponcooling.

In polymerizin the monomeric compounds, the usual methods of causing thepolymerization reaction to occur may be employed, including theutilization of catalysts in accordance with the processes well known inthe abrasive art. The monomeric esters polymerize readily unde theinfluence of heat, light and oxygen which may be conveniently suppliedby employing an oxidizing agent such as sodium, hydrogen or benzoylperoxide as catalysts. The peroxides are particularly suitable where itis desired to hasten the polymerization action since they may beincorporated in the liquid monomers and the speed of reaction can bestill further hastened by heating, although heat alone is sufiicient tobring about polymerization.

The particular polymers which are to be employed are determined by theuses to which the articles are to be put. All of these compositions areof the so-called "thermally reversible type as distinguished from theheat-hardening resins of the class represented by the well-known phenolformaldehyde condensation products. Consequently, they have theadvantage that the mix 01' any defective article can be reused becausethe binders can be repeatedly softened by heat and solidified by coolingwithout a change in their properties.

As has been stated, the properties of the monomers employed aredependent upon both the composition of the monomers employed and uponthe chain length of the polymers. Generally more elastic than those ofmethacrylate acid and have lower softening points, and likewise, theelasticity increases while the softening point decreases with increasein the number of carbon atoms in the alcohol employed to form the esterwith the acid. The softening point and th elasticity of the estersvaries from the secondary alcohols through the iso alcohols to thenormal alcohols in the order stated, the secondary alcohols having thehighest softening point and the lowest elasticity. The secondary butylalcohol ester has roughly the same properties as the methyl ester, whilethe propyl and other butyl esters are lower in softening point and moreelastic. Consequently, it is possible to obtain various degreesofelasticity and rigidity by the employment of difl'erent esters andsuch flexibility in the character of the binder is of very greatadvantage in the production of abrasive products, some of which shouldbe readily flexible, while others should be more rigid, depending uponthe use to which they are put. 4

Since the resinous polymers are softenable by heat, it is usuallydesirable to employ them in wet grinding operations where the productsare not allowed to become heated from the cutting operations. In suchapplications, these resinous polymers have been found to be highlysatisfactory as bonds for various types and sizes of abrasive grain.

I will now illustrate my invention with reference to specific examples,it being understood that such examples are for illustrative purposesonly and are not limitative.

Example I A methyl methacrylate polymer was dissolved in toluene to forman adhesive containing 40% solid resin. A coating of this adhesive inthe amount of 2.5 pounds of resin solids per ream of 480 sheets 9 inchesby 11 inches was spread on 130# rope cylinder paper. mesh fused aluminaparticles were electrostatically projected onto the adhesive coating.The coating of grain particles was dried sufficiently to set the grainsfirmly in position. The grain coating was then sized with a secondapplication of adhesive. The sizing adhesive, consisting of a 25%solution of methyl methacrylate polymer in toluene, was applied to thegrain coating at the rate of 3 pounds of resin solids per ream.

The abrasive coated paper was air dried to remove the toluene solventand finally baked 6 hours to complete the hardening of the adhesive.

Example [I A mixture of 70% methyl methacrylate polymer and 30% of veryfinely ground flint was pulverized to a fine powder. 80 mesh fusedalumina particles were moistened with a 20% solution of methylmethacrylate polymer in toluene. The resin-flint powder was then stirredinto the moistened grains and the solvent was removed to provide a massof resin coated abrasive particles. The amount of resin on the abrasiveapproximated 10%. The resin coated granules were uniformly distributedin a suitable steel mold and were compressed at a pressure of 2000pounds per square inch while heating the mold and its contents to 350 F.The molded article was cooled and removed from the mold and baked for 16hours at 300 F. to remove all traces of volatile materials.

As has been stated, my improved binders have the great advantage thatthey are obtainable in a wide variety of physical properties,particularly the properties of rigidity and elasticity, and it is thuspossible to prepare articles having desired combinations of propertieswhich have not been obtainable with the resinous binders most commonlyused heretofore; namely, the phenol formaldehyde condensation products.Furthermore, the binders may still be further modified by admixture withother binders such as phenolic resins, to raise the softening point andto increase the rigidity, or the usual modifications of admixing fillingmaterials or plasticizing agents may be used to still further vary theproperties of the binder material. The abrading characteristics ofabrasive articles are determined largely by the properties of thebinder, such as its elasticity, tensile strength and adhesiveness to theabrasive grains. The first two of these properties may be varied withina very considerable range by the selection of a suitable polymer or byadmixture with plasticizers or fillers. The resinous polymers of myinvention are all possessed of a high degree of adhesiveness which isemployment of binders of the class included within my invention providesabrasive articles having greatly improved properties for many purposes,

I and it will be seen that the methods which I have developed in theproduction of abrasive products with these binders have numerousadvantages over methods heretofore commonly used. For example, in theproduction of a resinous bonded abrasive with the phenol resins, thereis the problem resulting from the fact that gases are evolved from thecuring of the resins which has the tendency of making the binder spongy,and which require very careful heating of the abrasive article,particularly through the early stages of the reaction to prevent theentrapment of the gases in the binder with the consequent loss ofstrength and sponginess of the binder. In polymerizing the estersemployed in my invention, there is no such problem and consequently, the

process may be carried out without exercising the extreme degree of carenecessary where phenolic resins are employed.

This feature is of a considerable value in commercial operations whereprocesses which require careful control are likely to result inconsiderable spoilage with consequent loss. Furthermore, as

atrix of the has been stated, articles in which the binder isthermoplastic, as in the case of my improved binders, can be reclaimedand it is not necessary to discard the mixtures, if the abrasive articleis for any reason unsatisfactory, as is the case when heat-hardeningresins are used.

While I have described my invention in considerable detail in respect tocertain specific materials and steps of processes, it will be understoodthat this description is for the purpose of illustrating my inventionand is not limitative, the scope of the invention being defined by theappended claims.

I claim:

1. An abrasive article comprising abrasive grains and a bindercomprising a polymer in- A eluding a substantial proportion of an esterof an acrylic acid.

2. An abrasive article comprising abrasive grains and a bindercomprising a substantial proportion of polymethyl methacrylate.

3. An abrasive article comprising abrasive grains and a bindercomprising a substantial proportion of a co-polymer of an acrylic esterwith a vinyl ester.

4. An abrasive article comprising abrasive grains and a bindercomprising a substantial proportion of the co-polymer of an acrylic acidand an ester of acrylic acid.

5; An abrasive coatedarticle comprising a backing and a layer ofabrasive grains adhesively securedthereto by a binder comprising apolymer including a substantial proportion of an ester of an acrylicacid.

6. An abrasive coated article comprising a backing and a layer ofabrasive grains adhesively secured thereto by a binder comprising asubstantial proportion of the co-polymer of an acrylic acid and an esterof an acrylic acid.

7. In the process of making abrasive articles, the step of bondingabrasive grains with a bond comprising a substantial proportion of aresinous polymer of an ester of an acrylic acid.

NORMAN P. ROBIE'.

